The present disclosure relates to microelectromechanical systems (MEMS), and more particularly, to microbolometer package design.
Microelectromechanical systems (MEMS) are made up of one or more very small scale electrical components. For example, MEMS devices included in a MEMS can range in size from, for example, approximately 20 micrometers (μm) to approximately 1 millimeter (mm). Microbolometers are a type of MEMS device that include a thermal sensitive material having a temperature-dependent electrical resistance. One feature of microbolometers is the capability of measuring the power of incident electromagnetic radiation in response to receiving thermal or radiation energy. In essence, the microbolometer behaves as an image pixel, where the output intensity of the pixel is based on the amount thermal/radiation energy received. Accordingly, microbolometers are widely used in various energy sensing devices, such as infrared (IR) sensors, thermal imaging cameras, and night vision cameras, for example, to generate an image in response to thermal/radiation energy stimulation.
One design characteristic of microbolometers is the requirement of a high vacuum environment that thermally isolates the microbolometers from the external ambient temperatures, i.e., external thermal energy. Referring to FIG. 1, for example, a MEMS package 5 includes a microbolometer package 10. Microbolometer packages 10 typically include a vacuum region 15 created by sealing a wafer level package (WLP) 20 to a window cap wafer 25 (i.e., window lid 25) via a single narrow ring seal 30. The WLP 20 includes one or more reference pixels (RPs) 35 disposed thereon. Thus, the vacuum region 15 defines a clearance (d) between the RPs 35 and the window lid 25. However, the vacuum within the vacuum region 15 creates a pressure differential with respect to the exterior atmospheric pressure that causes the window lid 25 to deform/deflect toward the WLP 20 and into the vacuum region 15.
Conventional microbolometer packages 10 use only the single narrow seal ring 30 formed on a metal under-layer (U/L) between the WLP 20 and the window lid 25 to reduce stress and deflection of the window lid 25. For a 1D case we can treat this as a simple beam to understand how the deflection and stress at the joint are a strong function of the distance between the solder joints (span). The deflection in the window can be associated by the following formula:
                                          Y            max                    =                                                    -                                                                            w                      a                                        ⁢                                          l                      4                                                                            384                    ⁢                    EI                                                              ⁢                                                          ⁢              at              ⁢                                                          ⁢              X                        =                          L              /              2                                      ,        where                            (        1        )                Wa is force per unit length for the 1D case,    E is the young's modulus and is a material property of the window, and    I is the area moment of inertia, which can be further described as:
  I  =            bh      3        12  where h is the thickness of the window.
From equation 1, it can be appreciated that the deflection increases with the span (length) to the fourth power so that a small increase in span causes a much large increase in deflection. The bending moment on the joint is cause by the force that is exerted on the window. This scales directly with the area of the window. For the 1D case it scales directly with the span. Further, the pressure is defined as P=F/A, where “F” is force and “A” is area, and the pressure for is fixed at 1 atmosphere. However, the area “A” will change with the increase in the span. Thus, as the area “A” changes so does force “F”. Since there is a vacuum at vacuum region 15, the larger the span, the larger the force (F). The larger the force (F) that is creating the moment, the greater the stress also at the joint.
Referring to FIGS. 2A-2B, experimental results further illustrate that the pressure differential increases as the size of vacuum region 15 is increases, thereby increasing the deflection of the window lid 25. If the vacuum region 15 is formed too large, the window lid 25 is allowed to pivot about the seal ring 30 and can contact and/or crush the RPs 35 (see FIG. 1). One conventional solution to counteract the increase in deflection is to increase the thickness of the window lid 25. However, incident electromagnetic radiation is inhibited from reaching the WLP 20 as the window lid 25 thickness is increased. Consequently, conventional microbolometer package designs are limited to the size of the cavity region and the thickness of the window lid, which ultimately limits the overall thermal sensitivity and image quality of the imaging device.